You can always say "use" instead of "utilize" to avoid buzzwords and sound more conversational. In fact, there are only a few specific cases (mostly in science writing) where "utilize" is necessary:
http://www.quickanddirtytips.com/education/grammar/use-versu...
I use "use" when something is put to its intended or designed use; I use a shirt by wearing it. I utilize a shirt by wrapping it around my arm, as a pressure dressing, to stop bleeding.
From: Politics and the English language, George Orwell, 1946 https://www.mtholyoke.edu/acad/intrel/orwell46.htm
First, both are open-ended and relative. "Fast" and "efficient" compared to what? A cheetah is "fast", but it's not fast if I'm trying to use them to propel fighter jets. If I say, "my code is fast", am I saying it's fast enough for the problem at hand? Faster than it was before? Fast compared to most other code that solves the same problem?
"Efficient" has all of the same problems and it's also ambiguous. If my library is efficient, does that mean it runs fast? Starts up fast? Uses memory efficiently? Bandwidth?
"Performant" is etymologically strange, but in the place I've seen it used, it has an effective, precise meaning: "well-performing in speed relative to the needs of the problem". It means "fast enough" without the negative connotations of "enough", sort of like "plenty fast". Likewise, it's negation is usefully precise, "not performant" means "however fast it is, that isn't enough for our needs".
No, "performant" is one of those words which is usually chosen to flatter the speaker, not because it imparts some meaning which is so difficult to describe without it.
When it comes to technical topics, amateurs write to impress. Professionals write for clarity.